Since most of sheet metal forming process is done with a low tool velocity, it can be considered as a quasi-static process. Explicit dynamic FEM[1] has many advantages in treating severe nonlinearities such as nonlinear material, large deformation, instability and contact. In an explicit method, the time step is limited by the stability condition[2], so all solutions are essentially dynamic. Since contact algorithms used for explicit method[3] are more robust and straightforward than their implicit counterparts, explicit method are more attractive to simulate sheet metal forming [4,5]. Although there have been many researches on sheet metal forming analysis by an explicit method [4,5,[8][9][10][11][12][13][14], the dynamic effects are still not well-understood or quantified.To solve quasi-static problem in real time, a huge amount of computation time is necessary because of the small time step in an explicit method. Therefore when we use an explicit FEM for sheet metal forming analysis, it is conventional to convert the real problem to a virtual problem with a different time scale. In this context, time scaling and mass scaling techniques have been widely used in order to save computation time. If we use a large scaling factor with these methods, dynamic effects will increase and may cause incorrect results, especially for the stresses. In this situation, it is difficult to figure out how large the scaling should be to reduce computation time and still maintain the desired accuracy. The four node isoparametric shell element with one point integration with hourglass control is used by Belytschko and coworkers [6,7]. This shell element has been widely used to model sheet metal because of computational efficiency in explicit method.Several authors [5,8,9,13] have shown that the overall deformation and strain distribution can be predicted within acceptable accuracy if we use the scaling factor which yields a low ratio of total kinetic energy to total internal energy.

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